Dental resins, dental composite materials, and method of manufacture thereof

ABSTRACT

A low shrinkage, polymerizable oligomer comprises units of the structure:
 
AB  (I)
 
wherein A is an organic radical comprising 1 to about 6 (meth)acrylate groups and 0 to about 5 hydroxy groups; B is an organic radical comprising 1 to about 5 epoxide groups, and wherein A and B are linked through the reaction of an epoxide and a hydroxy group. In one embodiment, a dental restorative material comprises the low shrinkage, polymerizable dental oligomer, a filler system, and a curing system. These polymerizable dental resins may be used for a variety of dental materials, treatments, and restorative functions, including crown and bridge materials, fillings, adhesives, sealants, luting agents or cements, denture base materials, orthodontic materials and sealants, and other dental restorative materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/452,269filed Jun. 2, 2003, now U.S. Pat. No. 7,241,856, which is fullyincorporated herein by reference.

BACKGROUND

This invention relates to polymerizable dental resins for dentalcomposite materials and the method of manufacture of such resins forrestorative dentistry, and more particularly to dental compositematerials that are useful as crown and bridge materials either with orwithout an alloy substrate, as reconstructive materials, restorativematerials, filling materials, inlays, onlays, laminate veneers, dentaladhesives, cements, sealants and the like.

In recent years, materials used for dental restorations have comprisedprincipally of acrylate or methacrylate resins. Typical acrylic resinousmaterials are disclosed, for example, in U.S. Pat. No. 3,066,112 toBowen, U.S. Pat. No. 3,194,784 to Bowen, and U.S. Pat. No. 3,926,906 toLee et al. An especially important methacrylate monomer is thecondensation product of bisphenol A and glycidyl methacrylate,2,2′-bis[4-(3-methacryloxy-2-hydroxy propoxy)-phenyl]-propane (Bis-GMA).Alternatively, BisGMA may be synthesized from the diglycidyl ether ofbisphenol A and methacrylic acid (see U.S. Pat. No. 3,066,112 to Bowen).

Because the wear and abrasion characteristics and the overall physical,mechanical, and optical properties of these unfilled acrylic resinousmaterials is poor, and because acrylic resin systems exhibit highcoefficients of thermal expansion relative to the coefficient of thermalexpansion of the tooth structure, these substances by themselves areless than satisfactory. In particular, the disparity in thermalexpansion coupled with high shrinkage upon polymerization results inpoor marginal adaptability, and ultimately leads to secondary decay.Composite acrylic dental restorative materials containing acrylate ormethacrylate resins and fillers were thus developed, the fillersgenerally comprise inorganic materials based on silica, silicate basedglasses, or quartz. These filled compositions are useful for a varietyof dental treatments and restorative functions including crown andbridge materials, fillings, adhesives, sealants, luting agents orcements, denture base materials, orthodontic materials and sealants, andother dental restorative materials. Despite their suitability for theirintended purposes, however, many of these materials have shrinkages ofabout two to about 4% by volume upon polymerization.

Alternative resinous materials include the ring-opening polymerizationof epoxides. These resins have lower shrinkage than methacrylates, butexhibit compatibility problems with methacrylate bonding adhesives andcements when used together.

Epoxy/(meth)acrylate containing compounds containing both epoxy and(meth)acrylate functionality are also known and are obtained fromreaction of multi-epoxide containing compound with one or lessequivalent of (meth)acrylic acid, or reaction of hydroxyl containing(meth)acrylate with epichlorohydrin. Commercially availableepoxy/methacrylate include 3,4-epoxy-cyclohexyl methyl methacrylate fromDaicel Chemical, Japan. U.S. Pat. No. 6,187,833 to Oxman et al.generally discloses photocurable compositions containing an epoxy resin,a hydroxyl-containing material, and optionally a free radicallypolymerizable material. The compositions contain a ternaryphotoinitiator system comprising an iodonium salt, a visible lightsensitizer, and an electron donor compound. Oxman et al. disclose abifunctional epoxy/acrylate material, but do not disclose anepoxy/acrylate oligomeric material made from the reaction product of amulti-epoxide containing compound and hydroxy(meth)acrylate.

There remains a need in the art for dental resin materials that haveminimal shrinkage without sacrificing other advantageous physicalproperties. It is further desirable to improve other properties of thecured material such as fracture toughness.

SUMMARY

A polymerizable dental resin having low shrinkage upon polymerizationcomprises the polymerization product of an oligomer comprising a(meth)acrylate functionality and an epoxy functionality. In a preferredembodiment, the oligomer comprises units of the general structure (I):AB  (I)wherein A is an organic radical comprising 1 to about 6 (meth)acrylategroups and 0 to about 5 hydroxy groups; and B is an organic radicalcomprising 1 to about 5 epoxide groups, and A and B are linked throughthe reaction of an epoxide and a hydroxy group. The polymerizable dentaloligomer is conveniently synthesized by the selective reaction of amultifunctional epoxide with a hydroxy(meth)acrylate to yield areactive, polymerizable dental oligomer having an epoxy functionalityand an ethylenically unsaturated functionality.

In another embodiment, the polymerizable dental resin comprises thereaction product of a hydroxy(meth)acrylate of the formula

wherein m and n are independently integers of 1 to about 6; M is asubstituted or unsubstituted C₁ to C₃₃ alkyl or aryl group; and R ishydrogen or methyl; and a multifunctional epoxide of the formula

wherein E is a substituted or unsubstituted allyl, alkoxy, alkylether,heterocycle, alkaryl, or aryl group, and x is an integer of 2 to about6.

In yet another embodiment is a method of manufacturing a polymerizabledental resin comprising reacting, in the presence of a curing system,the above-described hydroxy(meth)acrylate; and multifunctional epoxide.

In another embodiment, a dental restorative material comprises the lowshrinkage, polymerizable dental resin comprising oligomer of structure(I), an optional filler system, and a curing system. In the formulationof dental restorative materials, both the epoxide functionality and the(meth)acrylate functionality can participate in the polymerization.These two functionalities can be activated simultaneously or onefunctionality may be activated selectively. The curing system can be aself-cure or a photocure system. The polymerizable dental resins may beused for a variety of dental materials, treatments, and restorativefunctions, including crown and bridge materials, fillings, adhesives,sealants, luting agents or cements, denture base materials, orthodonticmaterials and sealants, and other dental restorative materials.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It has unexpectedly been discovered that a polymerizable dental resinoligomer having both an epoxy functionality and a (meth)acrylatefunctionality has improved low shrinkage upon curing, together withimproved fracture resistance. Furthermore, it has been discovered thatthe use of a cationic initiator in the polymerization of the dentalresin is not necessary, although it may optionally be used. As usedherein, the term “(meth)acrylate” is intended to encompass both acrylateand methacrylate groups. The term “multifunctional epoxide” is intendedto encompass an organic compound comprising at least two epoxidefunctionalities. The term “hydroxy(meth)acrylate” is intended toencompass an organic compound comprising at least one hydroxyfunctionality and at least one (meth)acrylate functionality.

In particular, an unexpectedly improved polymerizable oligomer comprisesunits of structure (I)AB  (I)wherein A is an organic radical comprising 1 to about 6 (meth)acrylategroups and 0 to about 5 hydroxy groups; and B is an organic radicalcomprising 1 to about 5 epoxide groups, wherein A and B are linkedthrough the reaction of an epoxide and a hydroxy group. The generalstructure of (I) can have a variety of forms, for example A and B callbe in alternating order (e.g., ABAB . . . ) and/or branched. In oneembodiment, the oligomer has the form A_(a)B_(b) wherein a is an integerfrom 2 to 10, b is one, A is a monovalent radical, and B is a radicalhaving a valency corresponding to a. In another embodiment, a is 1, b isan integer from 2 to 10, A is a radical having a valency correspondingto b, and B is a monovalent radical.

The oligomer (I) is synthesized from the reaction of a multifunctionalepoxide and a hydroxy(meth)acrylate in the presence of a catalyst andheat. Preferably the amount of hydroxy groups in thehydroxy(meth)acrylate is less than one equivalent per equivalent ofepoxide. Depending upon the reaction conditions, such as ratio ofhydroxy to epoxy, the reaction temperature and time, and the amount ofcatalyst, the reaction product may comprise a variety of one or morecompounds, including the unreacted epoxides and hydroxy(meth)acrylates,the oligomer of structure (I), and a polymeric epoxy/(meth)acrylate orpolyepoxides resulting from the ring-opening of the epoxides.

Suitable multifunctional epoxides are compounds having two or moreepoxide (oxirane) functionalities, and include monomeric epoxy compoundsand epoxides of the oligomeric or polymeric type, which can bealiphatic, cycloaliphatic, aromatic, or heterocyclic. Thesemultifunctional epoxides may vary from low molecular weight monomericmaterials to oligomers to high molecular weight polymers and may varygreatly in the nature of their backbone and substituent groups, providedthat the backbone and the substituents thereon can be molecular groupsthat do not substantially interfere with the cure of the polymerizabledental resin at room temperature. Illustrative of permissiblesubstituent groups include halogens, ester groups, ethers, sulfonategroups, siloxane groups, nitro groups, phosphate groups, and the like.

The polymeric epoxides include linear polymers having terminal epoxygroups (e.g., a diglycidyl ether of a polyoxyalkylene glycol), polymershaving skeletal oxirane units (e.g., polybutadiene polyepoxide), andpolymers having pendent epoxy groups (e.g., a glycidyl(meth)acrylatepolymer or copolymer). These epoxides generally have, on average,greater than or equal to about two polymerizable epoxy groups permolecule. The “average” number of epoxy groups per molecule isdetermined by dividing the total number of epoxy groups in themultifunctional epoxide by the total number of epoxy-containingmolecules present.

The epoxides may be pure compounds or may be mixtures of compoundshaving greater than or equal to about two polymerizable epoxy groups permolecule. The number average molecular weight (M_(n)) of theepoxy-containing materials is about 58 to about 20,000 g/mole. Examplesof mixtures include two or more multifunctional epoxides havingdifferent number average molecular weight distributions ofepoxy-containing compounds, such as a low molecular weight (below 200g/mole) blended with an intermediate molecular weight (about 200 toabout 1,000 g/mole) and/or higher molecular weight (above about 20,000g/mole). Alternatively or additionally, the multifunctional epoxide maycomprise a blend of multifunctional epoxides having different chemicalnatures, such as aliphatic and aromatic, or functionalities, such aspolar and non-polar.

Useful multifunctional epoxides include those that contain cyclohexeneoxide groups such as epoxycyclohexanecarboxylates, typified by3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate;3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexanecarboxylate; and bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate.

Other multifunctional epoxides that are of particular utility in formingthe polymerizable dental resins include the formula (II)

wherein E is a substituted or unsubstituted alkyl, alkoxy, alkylether,heterocycle, alkaryl, or aryl group and x is an integer of 2 to about 6.Suitable substitutions on the E moiety include, but are not limited to,linear or branched, saturated or unsaturated C₁-C₁₂ alkyl; cyclic C₃-C₇alkyl; halogens; ester groups; ether groups; amide groups; aryl; and thelike.

In particular, the multifunctional epoxide may have the formula (III):

wherein Y is a divalent C₁-C₃₃ substituted or unsubstituted alkyl,alkoxy, aryl, alkylether, heterocycle, or alkaryl group, and q is 0 toabout 20. Preferably, Y is a divalent C₆-C₁₈ aryl or C₁-C₃₃ alkyl oralkylether-containing group, and q is an integer of 0 to about 10.Suitable substitution on the Y moiety include, but is not limited to,linear or branched, saturated or unsaturated C₁-C₁₂ alkyl; cyclic C₃-C₇alkyl; halogens; ester groups; ether groups; amide groups; aryl; and thelike.

A particularly preferred multifunctional epoxide is an aromaticdiglycidyl ether having the formula (IV):

wherein X is oxygen, sulfur, carbonyl, or a divalent C₁-C₆ alkyl,alkylether, or aryl group, d is an integer of 1 to 4, and i is aninteger of 0 to about 6. Preferably, X is a divalent alkyl oralkylether-containing group. Q is hydrogen or halogen, such as chlorine,bromine and iodine; and d is an integer of 2, 3, or 4. Preferably Q ishydrogen or bromine.

Further examples of suitable multifunctional glycidyl ethers are theglycidyl ethers of polyhydric phenols obtained by reacting a polyhydricphenol with an excess of a chlorohydrin such as epichlorohydrin, e.g.,the diglycidyl ether of 2,2-bis-(2,3-epoxypropoxyphenol)-propane(Bisphenol A); brominated diglycidyl ether of bisphenol A, thediglycidyl ether of Bisphenol F; the 1,4-butanediol diglycidyl ether ofphenolformaldehyde novolak (e.g., “DEN-431” and “DEN-438” from DowChemical Company); resorcinol diglycidyl ether (e.g., “KOPOXITE” fromKoppers Company, Inc.); and polyfunctional glycidyl ethers such as thediglycidyl ether of 1,4-butanediol, the diglycidyl ether of neopentylglycol, the diglycidyl ether of cyclohexanedimethanol, trimethylolethane triglycidyl ether, trimethylol propane triglycidyl ether, andmixtures comprising at least one of the foregoing ethers.

The hydroxy(meth)acrylate compounds used to synthesize the oligomer ofthe may contain a hydroxyl group terminally situated or pendent from apolymeric or copolymeric(meth)acrylate. A general structure of thehydroxy(meth)acrylate is shown in formula (V):

wherein m and n are independently integers from 1 to 6; M is asubstituted or unsubstituted C₁-C₃₃ alkyl or aryl group; and R ishydrogen or methyl. Suitable substitution on the M moiety include, butis not limited to, linear or branched, saturated or unsaturated C₁-C₁₂alkyl; cyclic C₃-C₇ alkyl; halogens; ester groups; ether groups; amidegroups; aryl; and the like.

A preferred hydroxy(meth)acrylate is a linear monofunctionalhydroxy(meth)acrylate wherein m and n equal 1, as shown in formula (VI):

Non-limiting examples of suitable hydroxy(meth)acrylates includecaprolactone 2-(methacryloyloxy) ethyl ester (CLMA); 2-hydroxyethylacrylate; 2-hydroxyethyl methacrylate (HEMA);3-hydroxypropyl(meth)acrylate; 4-hydroxybutyl(meth)acrylate;polyethylene glycol mono(meth)acrylate; glycerol di(meth)acrylate;trimethylolpropane di(meth)acrylate; pentaerythritol tri(meth)acrylate;and the (meth)acrylate of phenyl glycidyl ether. Blends of theaforementioned hydroxy(meth)acrylates can also be used to form thepolymerizable dental resin. The most preferred hydroxy acrylate orhydroxy methacrylate is CLMA and HEMA.

In one preferred embodiment, reaction of multifunctional epoxy (IV) withmonofunctional hydroxy(meth)acrylate (VI) yields a reaction productcomprising a mixture of products, including a polymerizable oligomerhaving the structure (VII):

wherein X, M, R, Q, d, and i are as defined above.

Further non-limiting examples of preferred polymerizable oligomersinclude the structures (VIII) and (IX):

wherein M is a divalent linear C₂-C₄ alkyl group, a divalent linearC₁-C₁₀ alkoxy group, e.g., (—OCH₂CH₂—)₁₋₁₀, or a C₂-C₁₀ divalent linearester group, e.g., —(CH₂)₄C(O)OCH₂CH₂.

In still another embodiment, the oligomer has the structure (X):

wherein Y is a divalent C₁ to C₃₃ substituted or unsubstituted alkyl,alkoxy, aryl, alkylether, heterocyclic, or alkaryl group; M is asubstituted or unsubstituted C₁-C₃₃ alkyl or aryl group; R is hydrogenor methyl; and q is 0 to about 20.

In yet another embodiment, the oligomer has the structure (XI)):

wherein m is 1 to 3, preferably 1; X is oxygen, sulfur, carbonyl, or adivalent substituted or unsubstituted C₁-C₆ alkyl or aryl group; i is 0to about 6; Q is hydrogen, chlorine, bromine or iodine; q is 0 to about20; M is a substituted or unsubstituted C₁-C₃₃ alkyl or aryl group; R ishydrogen or methyl; and d is 2, 3, or 4.

In the formation of the oligomers, the amount of hydroxy(meth)acrylateis selected so as to result in the polymerizable resin having a molarratio of epoxy:(meth)acrylate groups of about 1:10 to about 10:1,preferably about 1:5 to about 5:1, more preferably about 2:1 to about1:2. Suitable amounts may be readily selected by one of ordinary skillin the art, depending on the reactivity of the epoxide andhydroxy(meth)acrylate compounds, reaction conditions, and the like.Suitable reaction conditions are known to those of skill in the art.

The catalyst can be selected from those used in conventional cationic,anionic or coordination ring-opening polymerization. Preferred catalystsare metal organic catalysts comprising tin or titanium. Suitablenon-limiting examples of tin-containing catalysts are dibutyltindilaurate, dibutyltin maleate, dibutyltin diacetate, dioctyltin maleate,dibutyltin phthalate, stannous octoate, stannous naphthenate, stannousstearate, stannous 2-ethyl hexanoate, dibutyltin diacetylacetonate,dibutyltin oxide, and combinations comprising at least one of theforegoing tin based catalysts. Suitable non-limiting examples oftitanium-based catalysts are tetrabutyl titanate, tetrapropyl titanate,tetraisopropyl titanate, triethanolamine titanate, titaniumtetraacetylacetonate, and combinations comprising at least one of theforegoing titanium based catalysts. The preferred catalysts are stannousoctoate or stannous 2-ethyl hexanoate.

It is generally desirable to use the catalyst in an amount of about 0.10to about 10 mole percent (mole %) based on the total moles of thereactant mixture. Within this range it is generally desirable to utilizethe catalyst in an amount of greater than or equal to about one,preferably greater than or equal to about 2, and most preferably greaterthan or equal to about 3 mole % based on the total moles of thereactants. Within this range, it is generally desirable to utilize thecatalyst in an amount of less than or equal to about 8, and preferablyless than or equal to about 7 mole % based on the total moles of thereactants.

The above-described polymerizable dental resin can be used together witha curing system, other optional viscous resins, optional diluents,and/or an optional filler system to provide a dental restorativematerial for the formation of dental restorations. It is generallydesirable to use the above-described polymerizable dental resin in anamount of about 1 to about 99 weight percent (wt %) based on the totalweight of the dental restorative material. Within this range it isgenerally desirable to use the polymerizable dental resin in an amountof greater than or equal to about 10, preferably greater than or equalto about 30, and most preferably greater than or equal to about 50 wt %based on the total weight of the dental restorative material. Withinthis range, it is generally desirable to utilize the polymerizabledental resin in an amount of less than or equal to about 95, andpreferably less than or equal to about 90 wt % based on the total weightof the dental restorative material.

Known viscous resins may be added to the polymerizable dental resin toprovide a dental restorative material. Non-limiting examples includepolyurethane dimethacrylates (PUDMA), diurethane dimethacrylates(DUDMA), and/or the polycarbonate dimethacrylate (PCDMA) disclosed inU.S. Pat. Nos. 5,276,068 and 5,444,104 to Waknine, which is thecondensation product of two parts of a hydroxyalkylmethacrylate and 1part of a bis(chloroformate). Another advantageous resin having lowerwater sorption characteristics is an ethoxylated bisphenol Adimethacrylate (EBPDMA) as disclosed in U.S. Pat. No. 6,013,694. Anespecially useful methacrylate resin is the condensation product ofbisphenol A and glycidyl methacrylate,2,2′-bis[4-(3-methacryloxy-2-hydroxy propoxy)-phenyl]-propane (Bis-GMA).

Diluent monomers may be used to increase the surface wettability of thecomposition and/or to decrease the viscosity of the polymerizationmedium. Suitable diluent monomers include those known in the art such ashydroxy alkyl methacrylates, for example 2-hydroxyethyl methacrylate and2-hydroxypropyl methacrylate; ethylene glycol methacrylates, includingethylene glycol methacrylate, diethylene glycol methacrylate,tri(ethylene glycol)dimethacrylate and tetra(ethyleneglycol)dimethacrylate; and diol dimethacrylates such asbutanedimethacrylate, dodecanedimethacrylate, or1,6-hexanedioldimethacrylate (HDDMA). Tri(ethylene glycol)dimethacrylate(TEGDMA) is particularly preferred.

Diluent monomers or viscous resins, when present, are incorporated intothe dental restorative materials in an amount of about 1 to about 70 wt% of the total dental restorative material.

The optional filler composition may comprise one or more of theinorganic fillers currently used in dental composite materials.Preferred fillers include those, which are capable of being covalentlybonded to the low shrinkage, polymerizable dental resin matrix itself orto a coupling agent (e.g., silanes) that is covalently bonded to both.Examples of suitable filling materials include but are not limited to,silica, quartz, strontium silicate, strontium borosilicate, lithiumsilicate, lithium alumina silicate, amorphous silica, ammoniated ordeammoniated calcium phosphate, tricalcium phosphate alumina, zirconia,tin oxide, titania and combinations comprising at least one of theforegoing fillers. Some of the aforementioned inorganic fillingmaterials and methods of preparation thereof are disclosed in U.S. Pat.Nos. 4,544,359 and 4,547,531, pertinent portions of which areincorporated herein by reference. Organic-inorganic fillers of POSS™(Hybrid Plastics) can be incorporated into the composites as disclosedin co-assigned U.S. patent application Ser. No. 10/136,031. Otherorganic-inorganic fillers such as zirconium methacrylate and zirconiumdimethacrylate under the codes of CXZR050 and CXZR051 (Gelest, Inc.) canalso be used. Suitable high refractive index filler materials such ashigh refractive index silica glass fillers; calcium silicate basedfillers such as apatites, hydroxyapatites or modified hydroxyapatitecompositions may also be used. Alternatively, inert, non-toxicradiopaque materials such as bismuth oxide (Bi₂O₃), zirconium oxide,barium sulfate, and bismuth subcarbonate in micro- or nano scaled sizesmay be used.

Suitable fillers have particle sizes of about 0.01 to about 5.0micrometers, and may further comprise bound or unbound silicate colloidsof about 0.001 to about 0.2 micrometers. These additional fillers mayalso be treated with a silane-coupling agent to increase adhesion withthe low shrinkage, polymerizable dental resin. Commercially availablesilane treated fumed silica based on Aerosil A200 can be obtained fromDegussa Corp under the names of Aerosil R711 and R7200.

The amount of total filler composition in the dental restorativematerial can vary from about 1 to about 90 wt % based on the totalweight of the dental restorative material. The amount used is determinedby the requirements of the particular application. Thus, for example,crown and bridge materials generally comprise about 60 to about 90 wt %filler; luting cements comprise about 20 to about 80 wt % filler;sealants generally comprise about 1 to about 20 wt % filler; adhesivesgenerally comprise about 1 to about 30 wt % filler; and restorativematerials comprise about 50 to about 90 wt % filler, with the remainderin all cases being the polymerizable dental resin and other optionallyadded resins.

The low shrinkage, polymerizable dental resin may be used together witha curing system, which generally includes polymerization initiators;polymerization accelerators; ultraviolet light absorbers; antioxidants;and other additives known in the art.

Suitable polymerization initiators are those initiators, which can beutilized in UV-activated cure or visible light-activated curecompositions. For example, visible light curable compositions employlight-sensitive compounds, including but not being limited to benzil,benzoin, benzoin methyl ether, DL-camphorquinone (CQ), and benzildiketones. Either UV-activated cure or visible light-activated cure(approximately 230 to 750 nm) is acceptable. The amount ofphotoinitiator is selected according to the curing rate desired. Aminimal catalytically effective amount is generally about 0.01 wt % ofthe total resin compositions, and will lead to a slower cure. Fasterrates of cure are achieved with amounts of catalyst in the range fromgreater than about 0.01 percent to about 5 wt % of the dental compositematerial. The total resin composition is hereby defined as the totalweight of the polymerizable dental resin and other resinous materials,such as for example, resinous diluents, which are used in the dentalrestorative material.

Alternatively, the dental restorative material may be formulated as aself-curing system. Self-curing dental composite materials willgenerally contain free radical polymerization initiators such as, forexample, a peroxide in an amount of about 0.01 to about 1.0 wt % of thetotal resin dental composite material. Particularly suitable freeradical initiators are lauryl peroxide, tributyl hydroperoxide and, moreparticularly benzoyl peroxide.

Polymerization accelerators suitable for use are the various organictertiary amines well known in the art. In visible light curable dentalcomposite materials, the tertiary amines are generally acrylatederivatives such as dimethylaminoethyl methacrylate and, particularly,diethylaminoethyl methacrylate (DEAEMA) in an amount of about 0.05 toabout 0.5 wt % of the total dental composite material. In theself-curing dental composite materials, the tertiary amines aregenerally aromatic tertiary amines, preferably tertiary aromatic aminessuch as ethyl 4-(dimethylamino)benzoate (EDMAB),2-[4-(dimethylamino)phenyl]ethanol, N,N-dimethyl-p-toluidine (DMPT), andbis(hydroxyethyl)-p-toluidine. Such accelerators are generally presentin an amount of about 0.5 to about 4.0 wt % of the total dentalcomposite material.

It is furthermore preferred to employ an ultraviolet absorber in anamount of about 0.05 to about 5.0 wt % of the total dental restorativematerial. Such UV absorbers are particularly desirable in the visiblelight curable dental restorative materials in order to avoiddiscoloration of the resin from incident ultraviolet light. Suitable UVabsorbers are the various benzophenones, particularly UV-5411 availablefrom American Cyanamid Company.

In a preferred embodiment, in one manner of proceeding, thepolymerizable dental resin is prepared by reacting the multifunctionalepoxide with the hydroxy acrylate and/or hydroxy methacrylate in thepresence of a catalyst. The resulting polymerizable dental resin is thenformulated into a dental restorative material by mixing with the fillercomposition and the curing system and applying to the tooth to berepaired.

Alternatively, the dental restorative material may be formulated as atwo-part system, wherein the first part can comprise the low shrinkage,polymerizable dental resin, and the filler composition. The second partcan comprise the curing system and optional diluent monomers. Whennecessary, the two parts are metered out and then mixed using a spatula.The cure may be initiated through the use of UV light or by raising thetemperature of the mixture. The dental restorative material thusobtained is then placed in the tooth to be repaired after it isappropriately prepared. Methods for use of the above-describedcompositions are well known in the art.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

Samples of a bisphenol A monomer comprising an epoxy and a methacrylategroup (bisphenol A epoxy/methacrylate, “BAEM”) (Samples 1-3) orbisphenol F epoxy/methacrylate (BFEM) (Samples 4-6) were prepared byreacting either the diglycidyl ether of bisphenol A (DGEBA) or thediglycidyl ether of bisphenol F (DGEBF) with 2-hydroxyethyl methacrylate(HEMA), all obtained from Sigma-Aldrich, in the molar ratios of HEMAwith epoxy shown in Table 1.

The mixture was stirred using a magnetic stirrer. The flask wasmaintained in an oil bath at a temperature of 130-170° C. during thecourse of the reaction. The reaction was catalyzed by the addition of 5mole % (based on the total moles of the reactants) of stannous2-ethylhexanoate (SEH) also obtained from Sigma-Aldrich. The total timeof the reaction was from 2 to 8 hours. The reaction was monitored byFTIR and stopped when the intensity of C—O stretching in oxirane ring at910 cm⁻¹ did not decrease significantly.

TABLE 1 mole % of HEMA Sample No. to epoxy BAEM or BFEM 1 0.25 BAEM1 20.50 BAEM2 3 0.75 BAEM3 4 0.25 BFEM1 5 0.50 BFEM2 6 0.75 BFEM3

EXAMPLE 2

All the resin or resin combinations shown in Table 2 below were mixedwith 3 wt % diaryliodonium hexafluoro antimonite commercially availablefrom Sartomer Company, 0.3 wt % camphorquinone (CQ) obtained fromAldrich Chemicals Company and 0.2 wt % ethyl 4-(dimethylamino)benzoate(EDMAB) commercially available from Aldrich. Sample 11 is a comparativeexample and represents a blend of 70 wt % epoxy resin (DGEBA) with 30 wt% of an acrylate resin i.e., ethoxylated bisphenol A dimethacrylate(EBPADMA), while samples 12 and 13 represent a blend of thepolymerizable dental resin of this disclosure i.e., 70 wt % BAEM1 orBAEM2 resin with 30 wt % EBPADMA, wherein the wt % is calculated withrespect to the total weights of the respective BAEM resin and EBPADMA.

A small amount of the resin of each sample (0.2 grains) was placed in amixing well and was cured using a visible light source Cure-Lite™(commercially available from Pentron Corp.) for different time periods.Gel time is the time taken by the resin to reach an infinite viscosityand was determined using a spatula in the mixing well. Hardening time isthe time taken by the resin to attain a hardened mass felt by touchingwith a spatula.

TABLE 2 Sample No. Composition Gel Time Hardening Time 7 DGEBA 1 minute2 minutes 8 DGEBF 6 minutes 12 minutes 9 BAEM1 1 second 5 minutes 10BAEM2 1 second 2 minutes 11 DGEBA/EBPADMA 1 second 4 minutes (70/30 wt.ratio) 12 BAEM1/EBPADMA 1 second 30 seconds (70/30 wt. ratio) 13BAEM2/EBPADMA 1 second 12 seconds (70/30 wt. ratio) 14 EBPADMA 1 second12 seconds

As can be seen in Table 2, samples 9 and 10 obtained by the reaction ofDGEBA with HEMA reach the gel point much more rapidly than the samples 7and 8 obtained by reacting the epoxy precursors DGEBA and DGEBFrespectively. Sample 11, which represents a blend of an epoxy resin withan acrylate resin gels within 1 second as do samples 12 and 13, whichare blends of the polymerizable dental resin of this disclosure withEBPADMA. However, both samples 12 and 13 takes a much shorter time(approximately 30 seconds or less) to reach a hardened mass as comparedwith sample 11, which takes approximately 4 minutes. Thus blendscomprising the polymerizable dental resin can generally be cured in amuch shorter time period than the corresponding comparative blend.

EXAMPLE 3

Samples 15-17 were made by mixing BAEM1 with EBPADMA in different weightratios as indicated in Table 3. These samples were cured by utilizing acuring system comprising 3% wt % diaryliodonium hexafluoro antimonite(SarCat®CD 1012, Sartomer Corp.), 0.3 wt % camphorquinone (CQ) and 0.2wt % EDMAB where the percentages are calculated with respect to thetotal weight of the composition. Three point bending strength orflexural strength was measured on all samples using an ATS machine asper ISO 4049 for Resin Based Filling Materials (1997). The samples werecured for a total four minutes using visible light with CureLite™ Pluscuring box (Pentron Corp.) Samples were then trimmed and stored in waterat 37° C. for 24 hours before testing. The results are listed in Table3.

TABLE 3 Flexural strength in psi Sample No. Resin or resin combinations(standard deviation) 15 BAEM1/EBPADMA 18985 (941) (70/30 wt ratio) 16BAEM1/EBPADMA 18182 (1383) (50/50 wt ratio) 17 BAEM1/EBPADMA 18631(1128) (30/70 wt ratio) 18 EBPADMA 4571 (739)

Table 3 clearly shows that the blends containing the BAEM and EBPADMAhave superior flexural strength than those samples obtained by curingthe EBPADMA alone.

The low shrinkage, polymerizable dental resin or blends comprising thepolymerizable dental resin thus display a number of advantages overother resins used in dental composite materials. These resins or theblends comprising these resins generally display a shrinkage of lessthan or equal to about 8, preferably less than or equal to about 6, morepreferably less than or equal to about 4, and most preferably less thanor equal to about 2 volume percent upon curing as compared with thevolume occupied prior to curing. The polymerizable dental resins orblends comprising these resins also display a flexural strength greaterthan or equal to about 15,000, preferably greater than or equal to about16,000, more preferably greater than or equal to about 17,000, and mostpreferably greater than or equal to about 18,000 psi (pounds per squareinch) upon curing with the Cure-Lite™ curing unit for a time period ofabout 2 to about 5 minutes.

EXAMPLE 4

A dental composite containing an epoxy/methacrylate resins BAEM1 fromExample 1 and an ethoxylated₆ bisphenol A dimethacrylate available underthe trade designation CD541 from Sartomer in 50/50 wt % ratio wasprepared. The resin contains cationic and free radical initiators of 3wt % diaryliodonium hexafluoro antimonite (SarCat®CD 1012), 0.4% CQ and0.8% EDMAB. The paste is composed of 26% resin, 2% silane treated OX50(Degussa Corp.), 52% silane treated barium glass filler with an averageparticle sizes of 0.7 micrometers (Schott Glass) and 20% zirconiumsilicate filler. Shrinkage was measured using a mercury dilatometerdeveloped by NIST. The shrinkage of this composite is about 1.5% byvolume upon setting. As a comparison, a composite product availableunder the trade designation SIMILE™ (Pentron Corp.) with a similarfiller composition was also tested. The shrinkage of the SIMILE™composite is about 2.3% by volume.

EXAMPLE 5

A bromine-containing methacrylate/epoxy resin was synthesized from thereaction of HEMA or CLMA with brominated bisphenol A diglycidyl ether(BRDGEBA) using the same method as described in Example 1. The molarratio of hydroxyl group to epoxy was 0.5. The resultingmethacrylate/epoxy resin from HEMA and CLMA are abbreviated as BRBAEM1and BRBAEM2, respectively.

EXAMPLE 6

Resin combinations of BRBAEM2 and EBPADMA in 50/50 wt % ratio withdifferent initiation systems were prepared and their flexural strengthswere compared as shown in Table 4. In Sample 19, both cationicpolymerization of epoxy and free radical polymerization of methacrylatewere utilized. In Sample 20, only free radical polymerization ofmethacrylate was utilized.

TABLE 4 Sample No. Initiating System Flexural Strength (psi) 19 3%CD1012, 0.3% CQ, 15934 (791) 0.2% EDMAB 20 0.3% CQ, 0.2% EDMAB 15105(972)

Table 4 shows no difference between the strength of Sample 19 and Sample20. The addition of cationic photo initiator does not increase thestrength in this case.

EXAMPLE 7

Dental composite (Sample 21) containing an epoxy/methacrylate resinBREPMA2 and an ethoxylated₆ bisphenol A dimethacrylate available underthe tradename CD541 (Sartomer) in 50/50 wt % ratio was prepared.Shrinkage as well as strength was tested. As a comparison, a paste(Sample 22) containing a commercial resin system (SIMILE™, (PentronCorp.) having a combination of BisGMA/PCBisGMA/UDMA/HDDMA (each of 25%)was also prepared and tested. In both resin systems, no cationicphotoinitiator was added. Both resins contain free radical initiators0.3% CQ and 0.6% EDMAB. Both Samples 21 and 22 have 35 wt % resin, 10 wt% Aerosil R 7200 (Degussa) and 55 wt % zirconium silicate filler asabove. The modulus of rupture (MOR), an indicator of flexural strength,and shrinkage of these two composites are compared in Table 5. Shrinkagewas measured using a mercury dilatometer developed by NIST. Results areshown below in Table 5.

TABLE 5 Sample No. MOR (psi) Shrinkage 21 16023 (1624) 1.9 22 16197(1533) 2.8

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedembodiments.

1. A polymerizable dental resin composition comprising an oligomercomprising units having the structure:AB  (I) wherein A is an organic radical comprising 1 to about 6(meth)acrylate groups and 0 to about 5 hydroxy groups; and B is anorganic radical comprising 1 to about 5 epoxide groups, wherein A and Bare linked through the reaction of an epoxide and a hydroxy group; and acuring system consisting of a free radical cure initiator and a freeradical cure accelerator.
 2. The polymerizable dental resin compositionof claim 1, wherein the ratio of total epoxide groups to (meth)acrylategroups is about 1:10 to about 10:1.
 3. The polymerizable dental resincomposition of claim 1, wherein the ratio of total epoxide groups to(meth)acrylate groups is about 1:5 to about 5:1.
 4. The polymerizabledental resin composition of claim 1, wherein the ratio of total epoxidegroups to (meth)acrylate groups is about 1:2 to about 2:1.
 5. Thepolymerizable dental resin composition of claims 1, wherein the resinhas a volume shrinkage of less than about 8% after curing.
 6. Thepolymerizable dental resin composition of claim 1, wherein the resin hasa volume shrinkage of less than about 2% after curing.
 7. A curabledental restorative material comprising: the polymerizable dental resincomposition of claim 1; optionally, up to 90 weight % of a filler systembased on the total weight of the dental restorative material; and acuring system consisting of a free radical cure initiator and a freeradical cure accelerator.
 8. The dental restorative material of claim 7,further comprising about 1 to about 90 weight % filler based on thetotal weight of the dental restorative material.
 9. A method of making adental restoration, comprising applying to a site to be restored acomposition comprising a a curing system consisting of a free radicalcure initiator and a free radical cure accelerator; and an oligomercomprising units having the structure:AB  (I) wherein A is an organic radical comprising 1 to about 6(meth)acrylate groups and 0 to about 5 hydroxy groups; and B is anorganic radical comprising 1 to about 5 epoxide groups, wherein A and Bare linked through the reaction of an epoxide and a hydroxy group; andcuring the composition to form a dental restoration.